This invention relates generally to power sequencing for a circuit board, and in particular an apparatus and method for enabling hot swapped circuit boards to receive multiple power voltages from a backplane or a midplane and supply the multiple power voltages to devices on the circuit board in a predictable sequence.
Computer manufacturers are adopting standards, such as the Compact PCI Specification and the InfiniBand Technology Specification requiring a backplane to provide not just one voltage but multiple voltages to a peripheral card inserted into the midplane or backplane during active operation of a system. Providing multiple voltages from the backplane tends to increase the amount of power practically available to the peripheral card, because power is then no longer unnecessarily dissipated by converting from a voltage provided by the midplane or backplane to a voltage not provided by the midplane or backplane but required by the peripheral card, or circuit board.
However, providing multiple voltages from the backplane to the peripheral card can be problematic. Initially, the peripheral card may be inserted at a skewed angle into the backplane.
Such skew may prevent contacts on the peripheral card from connecting to the backplane in a predictable order. Mechanical skew may thus result in the multiple voltages provided by the backplane being supplied to the peripheral card in random sequences varying over several insertions of the peripheral card.
Devices such as the Intel 21554 and Intel 21555 (but also including virtually all integrated circuits available commercially) have been designed to require that they be powered up by certain voltages in a predictable sequence. If such devices receive those voltages out of order, the devices may behave unpredictably, or even be destroyed. A peripheral card including such devices must ensure that the multiple voltages provided from the backplane are supplied to the devices on the peripheral card in the required, predictable sequence. Thus, a solution to the power sequencing problem demands that the multiple voltages provided by the backplane be supplied to the peripheral card in a predictable sequence.
Moreover, providing multiple voltages from the backplane to the peripheral card involves other concerns. During operation, many devices require a certain voltage difference between the multiple voltages supplied to the devices. Further, one of the provided multiple voltages may be interrupted. While a board is being removed from a backplane or motherboard, voltages may be removed from the board out of sequence with respect to the requirements of an integrated circuit powered by the voltages. For example, the peripheral card might be physically removed from the backplane. In addition, contact bounce, or intermittent electrical connection between the backplane and the peripheral card, may occur during insertion or removal of the card. The resulting sudden swing from an expected power voltage to zero volts may harm the peripheral card. A power sequencing solution addressing the voltage difference and interrupted voltage concerns may help ensure reliable operation of devices on the peripheral card.
Another concern in the insertion or removal of the card is current limiting, to prevent large inrush currents as the peripheral card""s decoupling capacitance is charged, as the inrush currents may disrupt voltages supplied to other devices plugged into the midplane or backplane. Real estate on a peripheral card can be quite limited. The size or quantities of resistors needed to provide current limiting is frequently prohibitive. A power sequencing solution might also address the problem of inrush currents in a compact format.
To overcome the limitations described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a circuit board for use in systems that provide multiple power voltages to circuit boards. It is one object of the invention to provide a circuit board having circuitry with the property of receiving multiple power voltages generated externally from the circuit board and providing the multiple power voltages in a predictable sequence to a part of the circuit board. It is a further object of the invention to provide a highly compact power sequencing solution which also implements current limiting, and has minimal space and cost impact on the circuit board. It is another object of the invention to prevent damage to the circuit board in the event that the supply of multiple power voltages to the circuit board should be interrupted.
In accordance with these objects, the present invention is a circuit board comprising an electrical connector, a sequencing circuit, and an element. An element is an electronic device that receives one or more voltages in a predictable sequence, either at start up or during operation. Multiple power voltages are generated externally from the circuit board. The electrical connector of the circuit board receives the multiple power voltages from a second circuit board, such as a backplane or a motherboard. The sequencing circuit receives the multiple power voltages from the electrical connector. When a part of the sequencing circuit receives a signal indicating that the sequencing circuit has received a first power voltage, the sequencing circuit provides a second power voltage to the element electrically coupled to the sequencing circuit. The sequencing circuit may implement current limiting to conserve space on the circuit board.
The circuit board of the present invention is utilized in a computer system including a memory, the circuit board, and a second circuit board. The second circuit board may be a midplane, backplane or motherboard. The second circuit board provides the multiple power voltages to the circuit board.
The invention also includes a method for supplying multiple power voltages to the circuit board from the second circuit board in a predictable sequence.
In a preferred embodiment, the circuit board of the present invention includes a clamping circuit and a discharge circuit. The clamping circuit includes multiple sequentially coupled diodes to limit a voltage difference between the first power voltage and the second power voltage provided by the sequencing circuit to the element, for example when the motherboard or backplane does not provide one of the first power voltage and the second power voltage to the sequencing circuit. The discharge circuit includes a diode to limit a voltage difference between the first power voltage and the second power voltage when one or both of the first power voltage and the second power voltage are interrupted briefly or for a long time. For example, one or both of the first power voltage and the second power voltage may become interrupted briefly during contact bounce, and may become interrupted for a long time when the circuit board is removed from the second circuit board providing the multiple power voltages.
Another embodiment solving the power sequencing problem includes a design limiting current by means of a transistor and a current sensing circuit. This compact power sequencing solution implements any necessary current limits in addition to power sequencing.
The sequencing circuit is compact, and does not consume much real estate on the circuit board. The invention is cost efficient and and can be implemented as a step in the board fabrication process.